The advantageous influences which electric fields can have on combustion flames are known. According to the publications
Industrial and Engineering Chemistry 43 (1951), pages 2726 to 2731,
12th Annual energy-sources technology conf. (1989), pages 25 to 31 and
AIAA Journal 23 (1985), pages 1452 to 1454
the effects of the electric field reside in an improvement to the flame stability. According to
Combust. Flame 78 (1989), pages 357 to 364 and
Combust. Flame 119 (1999), pages 356 to 366
the carbon emissions are reduced. Further, according to
Fossil Fuel Combustion, ASME 1991, pages 71 to 75 and
Fluid Dynamics 30 (1995), pages 166 to 174 the emission of gaseous pollutants is reduced.
It is also known from Combust. Flame 55 (1984), pages 53 to 58 to influence combustion operations by electric discharges, in particular corona discharges. In this case too, the flame stability can be improved and the pollutant emissions can be reduced. Technical applications of the abovementioned effects are described in WO 96/01394 A1, U.S. Pat. No. 3,416,870 A and U.S. Pat. No. 4,111,636 A.
A common feature of the known methods is that the electrodes which are required in order to generate the electric field or a discharge in the flame, are arranged in such a manner that the flame is either located between the field-generating electrodes or is surrounded by one electrode. This electrode can be identical to the combustion chamber. An arrangement of this nature is illustrated with reference to FIG. 1 of the description. In any event, it is possible to draw a straight connecting line between electrodes of opposite polarity in such a manner that the connecting line passes through the flame which is to be influenced.
In FIG. 1, the direction of propagation of a flame 2 or the direction of flow of the exhaust gases is indicated as the z direction. The location z=0 is determined by the position at which the solid, liquid or gaseous fuel is transformed into the flame. No significant ionization caused by the combustion process occurs at locations z<0.
Arrangements which correspond to the known art include at least one electrode or one or more parts of such an electrode extending exclusively or predominantly over areas where z>0. In this case, it is also possible for the combustion chamber which surrounds the flame to be an electrode or part of an electrode. In extreme cases, the arrangement is such that partial areas of the flame may touch an electrode. In any event, it is possible to draw a straight connecting line from one electrode to an electrode of opposite polarity in such a way that the connecting line passes through the flame.
One drawback of the known art described above is that the electric field which is generated by way of the electrodes passes through a large area of the flame, while the actual effect of the electric field occurs in what is known as the flame front. The flame front is a narrow area, compared to the dimensions of the flame, between the cold fuel and the flame in which the chemical reactions leading to the formation of the flame take place. Since the flame has an electrical conductivity which is not negligible, on account of the charge carriers contained therein, the fact that the electric field passes through wide areas of the flame indicates that an electric current flows throughout the flame area which is enclosed by the electrodes. This causes an increased energy consumption without contributing to the desired effect within the flame front. This is the case in particular if electrically conductive areas of the flame or its surroundings are in direct contact with the electrodes.